This invention relates to the digital communication art and, more particularly, to a means for reducing the length of digital data streams by concatenating sync bits in place of burst error correction, reference level parity bits.
Digitally encoded communication systems are well known in the art. In such systems, an information bit stream is processed whereby a data bit stream is produced. In convolutional encoding systems, for example, parity bits are derived from the information bits and the data bit stream comprises the iterative transmission of information and parity bits. Such convolutional encoding schemes may correct for either random errors or burst errors, or both. A random error is, by definition, of short duration affecting a limited number of bits. Burst errors, however, may affect an entire sequence of bits. In convolutional encoding systems which correct for burst errors of length kB, the encoding requires a series of kB reference level, or "zero" parity bits at the beginning of the data bit stream, and a similar sequence of jK zero parity bits at the conclusion thereof, where k and j are predetermined integers. Another processing scheme uses time diversity data transmission. In this scheme the same information bits are transmitted twice, with a given number B, of zero level bits transmitted therebetween. In addition, some systems incorporate selective calling, continuous transmission. Here, a series of messages of a known length are combined for a continuous transmission.
For all such digitally encoded transmission systems, it is necessary that the receiver contain a decoder for recovering the information bit stream from the data bit stream. It is essential that the decoder recognize a desired bit stream, and operate in synchronism therewith, for proper decoding. Commonly, preamble or sync bits are placed before the encoded message by the encoder thus notifying the decoder that a transmission is in progress. If there are n sync bits preceding m data bits, a total (n+m) bits are required, which requires (n+m) bits of transmission time. It is, of course, desirable to reduce transmission time to an absolute minimum.